Electromagnetic interference (EMI) is the impairment of a wanted electromagnetic signal by an electromagnetic disturbance. Each electronic component operates using a flow of charge carriers, the flow inducing a surrounding electromagnetic field. This surrounding electromagnetic field may cause sufficient noise within surrounding electronic components that the performance of the surrounding electromagnetic components is degraded. EMI is a particular problem in cellular phones in which an antenna emits radio frequencies (RF) which could affect the other circuitry operating in the phone. In order to isolate an electronic component from EMI, EMI shielding has been recently developed. Lin (U.S. Pat. No. 5,436,203) discusses an EMI shielded device. FIG. 1 is a cross-sectional view of a similar EMI shielded device. In FIG. 1, an electrically insulating encapsulant 38 mechanically protects a semiconductor die 32. The electrically insulating encapsulant 38 is constrained by a dam structure 40 so as not to encapsulate electrically conductive reference pads 18. Reference pads 18 are electrically connected to the reference plane 22 by electrically conductive vias 20. An electrically conductive encapsulant 42 is dispensed over the first encapsulant and is in contact with the reference pads 18. The electrically conductive encapsulant 42 is constrained by a second dam structure 44.
The shielding of device 32 requires the fabrication of two separate dam structures 40 and 44. This requires two separate process steps, one step for the deposition of each dam structure 40 and 44. Furthermore, the size of the package 30 is much larger than the size of the device 32 because a dam structure 40 must surround device 32 while a separate dam structure 44 surrounds the dam structure 40. This double dam structure results in a large footprint of package 30 on a printed circuit board.
It is desirable to reduce the number of process steps in packaging a semiconductor device. It is also desirable to reduce the package size relative to the device size.